1. Field of the Invention
The present invention relates to a time control circuit for a backlight inverter applied to displays such as LCD. More particularly, the invention relates to a time control circuit for a backlight inverter capable of controlling an initial startup time, a soft start time and a shutdown time via an externally connectable capacitor for time-setting, thereby decreasing the number of pins connected to the capacitor and consequently reducing manufacturing costs.
2. Description of the Related Art
In general, with an increasingly bigger market for LCD TVs and LCD monitors, a greater number of lamps have been mounted on a backlight unit. Also, increasingly larger and longer tubes of the lamps have diversified methods for driving the lamps.
Especially, lately, to lower price of the applications, vast attentions have been drawn on ways to save costs for manufacturing a backlight inverter.
FIG. 1 is a configuration diagram illustrating a backlight inverter according to the prior art. Referring to FIG. 1, the conventional backlight inverter includes a driver 11, a transformer 12, a lamp error detector 14 and a controller 15. The driver 11 generates a square wave signal based on detected lamp current. The transformer 12 raises the voltage of the square wave signal from the driver 11 and converts the signal into an alternating current signal to output alternating current necessary for lamp operation to the lamp 13. The lamp error detector 14 has a feedback node connected to a cold terminal of the lamp 13 to detect current (or voltage) traveling through the lamp 13 and detecting lamp errors such as openness of a voltage lamp based on the detected value to provide a protective signal. The controller 15 controls the operation of the driver 11 based on detected voltage of the lamp error detector 14, and controls the deactivation of the driver 11 in response to the protective signal. Herein, an input terminal of the lamp with high current supplied is referred to as a hot terminal whereas an input terminal of the lamp with low current supplied is referred to as a cold terminal.
In this backlight inverter, the controller 15, typically configured as an inverter control IC, controls an initial startup time T1 at power-on, then controls a soft start time T2, and a shutdown time T3 at an input of a protective signal. To control such three times T1, T2 and T3, the conventional time control circuit for the backlight inverter is configured as shown in FIG. 2.
FIG. 2 shows a time control circuit of a backlight inverter according to the prior art.
The conventional time control circuit of the backlight inverter includes first to third comparator elements 21 to 23. The first comparator element 21 belonging to the controller 15 has an inverse terminal connected to a first capacitor C1 through a first pin P1 and a non-inverse terminal connected to a first reference voltage Vr1 (about 0.3V) for controlling the initial startup time T1 to compare voltages of the both terminals to provide an initial startup signal SI. The second comparator element 22 has an inverse terminal connected to a second capacitor C2 through a second pin P2 and a non-inverse terminal connected to a second reference voltage Vr2 (about 0.7V) for controlling a soft start time T2 to compare voltages of the both terminals to provide a soft start signal SSS. The third comparator 23 has a non-inverse terminal connected to a third capacitor C3 through a third pin P3 and an inverse terminal connected to a third reference voltage Vr3 (about 1.5V) for controlling a shutdown time T3 for lamp errors such as open lamp to compare voltages of the both terminals to provide a shutdown signal SSD.
Also, the time control circuit includes first to third current sources IS1 to IS3 connected between the non-inverse terminals of the respective comparators 21 to 23 and power Vcc terminals; first to third zenor diodes ZD1 to ZD3 connected between the non-inverse terminals of the respective comparators 21 to 23 and a ground; and a protective switch SW connected between the third current source IS3 and the non-inverse terminal of the third comparator element 23 for switching on in response to a protective signal.
FIG. 3 is a timing chart illustrating major signals of FIG. 2.
Referring to FIG. 3, in a system employing the time control circuit for the backlight inverter of FIG. 2, at power-on, a high-level signal is inputted from the first comparator element 21. Then, current from the first current source IS1 connected to the power Vcc is charged on the first capacitor C1. If the voltage charged on the capacitor C1 reaches the first reference voltage Vr1 of 0.3V or more, the first comparator element 21 outputs a low level signal. Through this process, the initial startup time T1 is set during a predetermined duration (e.g., 10 msec) from the time point of power-on.
In addition, in case of power-on in the system, the second comparator element 22 outputs a high level signal. Then current from the second current source IS2 by the power Vcc is charged on the second capacitor C2. If the voltage charged on the second capacitor C2 reaches the second reference voltage Vr2 of 0.7V or more, the second comparator element 22 outputs a low level signal. Through this process, a soft start time T2 is set during a predetermined duration (e.g. 23 msec) from the time point of power-on.
Further, in the system adopting the time control circuit of FIG. 2, if an error signal SOL of lamp operation such as open lamp is inputted, a switch SW is on. Thereby, current from a third current source IS3 is charged on the third capacitor C3. If the voltage charged on the third capacitor C3 reaches a third reference voltage Vr3 of 1.5V or more, the third comparator 23 outputs a high level signal. Through this process, a shutdown time T3 is set.
In this fashion, the conventional time control circuit for the backlight inverter requires the aforesaid three kinds of time controls to drive a backlight such as CCFL. To achieve such three kinds of time controls, respective capacitors are coupled to respective three pins.
However, since the conventional time control circuit for the backlight inverter requires three externally connectable capacitors for the three time controls and three connecting pins for connecting three capacitors, production costs are increased, thus disadvantageously leading to higher production costs of the applications.